The invention relates to an optical detection device, especially for chemical analyses of small-volume samples, comprising at least one light source for emitting detection light, at least one photoelectric detection unit for detecting a light intensity and converting the light intensity into a corresponding electrical signal, at least one measuring cell for holding a sample to be examined, and one or more optical paths coupled to the at least one measuring cell being formed between the light source(s) and the photoelectric detection unit(s).
It has long been known to carry out qualitative and quantitative chemical analyses of samples by optical means. Examples of such optical measuring methods are electrophoresis and chromatography. In such an optical examination of a sample, detection light emitted by a light source impinges on the sample located in a measuring cell. The light leaving the measuring cell is detected by a photoelectric detection device. When the detection light interacts with the sample, or rather with an analyte contained in the sample, given a suitable absorption spectrum of the analyte, absorption of the detection light can occur and, if the analyte is capable of luminescence, for example as a result of having been suitably prepared with a fluorescence marker, the absorbed detection light can be emitted again by the analyte in the form of luminescence.
For modern biochemical diagnostics there is a general trend towards miniaturization of such optical detection devices so that the use of as small a quantity of sample as possible suffices. Furthermore, in medicine and biochemistry, samples are often examined in respect of several different analytes, so that it is necessary in the course of a rapid processing operation to examine qualitatively, and, where applicable, quantitatively, as far as possible all of the analytes simultaneously.
To examine a sample in respect of various analytes it is known, for example, to bring the sample into contact with a corresponding number of sensor layers, the sensor layers being selectively provided with chemical or biochemical recognition elements immobilized in the sensor layer. The recognition elements each comprise specific affinity partners of the relevant analyte to be detected.
For the optical detection of a specific analyte in the sample it is known, for example, to label the analyte to be detected, which will be captured by the recognition element sensitive thereto which is immobilized on the sensor layer, using a luminescent dye and to detect optically as a measured variable the luminescence radiation or the change in the luminescence radiation of the detection layer resulting from the contact between the analyte and the recognition element.
In the case of optical sensor devices, it is often possible to use the evanescent luminescence excitation method. In that method, excitation light is coupled into a waveguide surrounded by media of lower refractive index. The excitation light is guided in the waveguide by total reflection at the transition between the media having differing refractive indexes. However, in the total reflection, the excitation light enters a short distance into the adjacent medium, with an exponential reduction in its intensity, where it produces the so-called evanescent field. Using the evanescent light intensity, a sample directly adjacent to the optical waveguide can be excited to emit fluorescence. The sensor layer which is provided with the immobilized recognition elements and over which the flowable sample is passed is arranged on the optical waveguide. In such optical sensor devices, the optical waveguide is advantageously in the form of a planar optical waveguide. A planar optical waveguide of that kind on the one hand may be an integral component of a flow cell and serve, for example, as a cover plate for the flow channel and, on the other hand, it can be manufactured simply, and in a manner suitable for mass production, by known deposition methods.
If a large number of analytes is to be examined, it is expedient to arrange the individual biochemical sensor elements in an array. A light source and at least one light detection device are associated in each case with that array of sensor elements. In order to meet the requirements for a small design of the optical detection device, recourse is therefore had to individual edge-emitting semiconductor lasers and conventional semiconductor photodetectors. Such a device comprises, for example, an array of edge-emitting semi conductor lasers mounted on the surface of a substrate, the emission light of which is coupled into respective associated waveguides. The waveguides, which form the interaction zone with the sample, are in contact with the sensor layers provided with recognition elements specific for the relevant analytes. After passing along the interaction zone, the light can be guided via coupling-out devices onto the detection surface of respectively associated semiconductor photodetectors.
The edge-emitting semiconductor lasers are not as a rule, however, produced on the same substrate as the semiconductor photodetectors. Since edge-emitting semiconductor lasers according to conventional production technology emit light parallel to the surface of the substrate, it is necessary either to expose a side edge of a laser element inside the substrate, for example by etching a trench, and guide the emitted light out of the depths of the substrate via deflectors, or to remove the laser unit from the substrate. Since the deflectors in question can be produced only with great difficulty, edge-emitting semiconductor laser elements are usually removed from the substrate and mounted in the desired emission direction on the foreign substrate containing the semiconductor photodetectors. Despite these considerable limitations resulting from the high number of, in some cases, non-automatable operations in the manufacturing of such detector arrangements, in comparison with other laser systems, such as helium-neon lasers, edge-emitting lasers are unrivalled in terms of the space they require and also in their efficiency in converting electrical energy into optical energy, which, in the case of edge-emitting semiconductor lasers, is considerably greater than that of, for example, a helium-neon laser.
However, those known devices have the disadvantage, that there are limits to the increasing miniaturization, inasmuch as, even using edge-emitting semiconductor lasers that have been separated from their mother substrate and applied to a foreign substrate, the surface area occupied by an edge-emitting semiconductor laser is typically 300xc3x97100 xcexcm2. Owing to the need to separate the edge-emitting semiconductor laser from the mother substrate a and fasten it in a suitable orientation to a foreign substrate, the manufacturing process for an optical detection device according to the difficult and time-consuming and requires manual work, which adds considerably to the costs of the optical detection device.
The purpose of the invention, therefore, is to provide an optical detection device, especially for chemical multiple analyses, preferably of small-volume samples, that has a reduced minimum overall size and a simplified manufacturing process.
That problem is solved according to the invention in a first solution by an optical detection device of the kind previously mentioned that is further distinguished by the fact that each light source is a surface-emitting semiconductor laser.
The surface-emitting laser of the optical detection device according to the invention has, for example in current designs, a size of approximately 10xc3x9710 xcexcm on the substrate, with the result that the surface area occupied can thereby be reduced by a factor of about 1:300 in comparison with a customary commercially semiconductor laser of the kind previously mentioned at the beginning. Furthermore, a surface-emitting laser has a lower power consumption since the threshold currents in that component are lower by about an order of magnitude than in conventional edge-emitting laser diodes. Especially in the case of an array of a large number of detection devices according to the invention on a single substrate, this leads to appreciable easing of the requirement to cool the laser elements.
Compared with the known edge-emitting laser, the surface-emitting laser has the advantage in an optical detection device that, owing to the symmetrical and Gaussian beam profile of the surface-emitting laser, improved beam control and utilization are possible in the down-stream optical elements. Whereas edge-emitting semiconductor lasers generally have an elliptical beam geometry with various and relatively high divergences in the region of up to 30xc2x0, the surface-emitting semiconductor laser offers a considerably improved beam quality with a very small divergence of only about 5xc2x0 half-angle, so that smaller and simpler optical elements can be used.
The surface-emitting semiconductor lasers can be deposited in the form of process- and product-matched stacks of layers of differing stoichiometry in a simple and inexpensive manner by means of known production processes using molecular beam epitaxy or metallo-organic vapour-phase epitaxy methods. Circuits for driving the surface-emitting lasers, other electrical components and also the photoelectric detectors can furthermore be produced on the same substrate. This eliminates the previous is complicated and time-consuming operations of separating, aligning and applying the edge-emitting semiconductor lasers in the chosen array geometry to a foreign substrate which had to be carried out by hand. Rather, the use of surface-emitting semiconductor lasers makes it possible to adopt the technology for the production of integrated circuits which has matured to a high precision. Finally, the surface-emitting semiconductor lasers also have the further advantage of wavelength tuning as a function of their drive current, without longitudinal mode jumps occurring.
In a preferred embodiment of the invention, a plurality of surface-emitting semiconductor lasers is provided on a common substrate. As a result, recourse can be had to the extremely precise methods used in the manufacture of integrated semiconductor circuits for aligning the individual light sources with one another. In particular, the surface-emitting semiconductor lasers have a surface area of approximately 100 xcexcm2 on the substrate, so that a massive increase in the packing density of the light sources on the substrate by a factor of approximately 1:300 is possible compared with arrangements having edge-emitting lasers. This enables the optical detection device to be miniaturized, on the one hand, while also enabling a greater number of analytes to be investigated simultaneously.
It is also especially advantageous within the scope of the present invention to design the surface-emitting semiconductor lasers to be suitable for emission of visible light since, as a result, in addition to absorption measurements, the field of fluorescence analysis will also be accessible. Owing to the ability of the surface-emitting semiconductor lasers to be tuned as a function of the drive current, absorption and fluorescence spectroscopy is possible over a wavelength range of several nanometers. Owing to the short resonator length of the surface-emitting semiconductor laser, jumps in the longitudinal mode of the emitted laser light are eliminated, in contrast to conventional, edge-emitting laser diodes, thereby substantially increasing the reliability of the scanning of the accessible wavelength range.
In an especially advantageous manner, in the optical detection device according to the invention, the surface-emitting semiconductor laser is constructed in such a manner that, in the Bragg mirrors of the surface-emitting semiconductor laser, the concentration of the stoichiometric composition of adjacent layers of the multi-layer structure varies in a continuous, especially linear, manner. As a result of that linear graduation of the concentration transitions between the layers, which is also called xe2x80x9cgradingxe2x80x9d, the electrical resistance loss of the surface-emitting semiconductor laser for the drive current is reduced. Therefore, a lower power loss being converted into heat occurs, with the result that it is possible, on the one hand, to obtain a higher conversion ratio of electrical power into light power and, on the other hand, to reduce the hitherto-known problem of the substrate becoming heated when several surface-emitting semiconductor lasers integrated on the same substrate are operated simultaneously.
It is also advantageous in the optical detection device according to the invention for the surface-emitting semiconductor laser on the substrate to be defined in its lateral dimension by mesa etching, especially the surface and the flanks of the mesa-etched surface-emitting semiconductor laser being covered by a metal layer, leaving an emission window free for the light emission which is oriented perpendicularly to the substrate surface. This metal layer, which at the same time serves as a metal connection to the surface-emitting semiconductor laser to supply the drive current, causes, by extending over the entire mesa structure, improved heat removal, so that the surface-emitting semiconductor laser can be operated at higher drive currents and hence, a higher maximum light output power can be obtained.
The above-mentioned problem underlying the present invention is solved according to the invention in a second solution by an optical detection device, especially for chemical analyses of small-volume samples, comprising a plurality of light sources for emitting detection light, a corresponding plurality of photoelectric detection units, each associated with a corresponding light source, for detecting a light intensity and converting the light intensity into a corresponding electrical signal, and at least one measuring cell for holding a sample to be investigated, optical paths each interacting with the at least one measuring cell being formed between the light sources and the corresponding photoelectric detection units, wherein the plurality of light sources comprises at least one linear arrangement of edge-emitting semiconductor lasers produced on a common substrate. This second solution according to the invention has the advantage that the semiconductor lasers are produced and installed in the optical detection device in the form of line arrays, whereby the individual semiconductor laser elements within the line are aligned with one another very exactly by virtue of the photolithographic production processes Furthermore, the laborious separation and separate mounting of individual semiconductor elements on a foreign substrate which was customary according to the prior art is considerably simplified and rationalized in this second solution according to the invention, since the edge-emitting semiconductor lasers are formed in lines as one-piece elements comprising 100 or more integrated edge-emitting lasers in a single separating step.
It is also advantageous for the semiconductor laser(s), the photoelectric detection unit(s) and the optical path(s) coupled to the at least one measuring cell to be respectively provided on a first, second and third substrate which is in each case substantially planar. In particular, the first, second and third substrate can be stacked on top of one another. That stacked structure makes possible a modular construction of the optical detection device, with simple and precise alignment of the light sources, the photoelectric detection units and the measuring cells with one another. Owing to this modular construction it is possible to replace the measuring cells from one measurement to the next with a set of fresh measuring cells in a simple manner and without difficulty when the recognition substance immobilized on the sensor layers of the measuring cells has been consumed by a measuring process. It is thereby possible also to achieve cost savings in the measuring process, since the light source arrangement and the arrangement of optical photodetectors are reusable, and assembly of the optical detection device according to the invention is possible in everyday practice without great effort and, as a rule, without the need for special procedures. The various substrates advantageously have registration structures or marks produced by auto-aligning processes. The auto aligning processes comprise, for example, using identical masks in a lithographic process for production of the registration structures or marks.
In another advantageous embodiment, each optical path comprises a waveguide coupled to the at least one measuring cell. The waveguide coupled to the measuring cell is advantageously a monomodal waveguide or a waveguide carrying only few modes and/or a waveguide having a very high refractive index produced, for example, by metal oxides, especially titanium dioxide and tantalum pentoxide. To form a chemical sensor, a chemical coating is applied to the waveguide. Waveguides that are monomodal or that carry only few modes are distinguished by an especially high degree of sensitivity while being as small as possible. That degree of sensitivity is not as a rule achieved by multimode waveguides of planar construction.
Advantageously, at least two planar, separate, preferably inorganic, dielectric waveguides are constructed on a common carrier material to form a sensor platform. A sensor platform of this kind, which is ideally suited for use with an integrated or hybrid semiconductor laser and photodetector array, makes possible a parallel evanescent excitation and detection of the luminescence of identical or different analytes. The separate waveguides may each contain one or more coupling gratings.
A considerable advantage of the sensor platform is that, for example, several sample solutions can be analyzed simultaneously with a high degree of sensitivity. No washing or cleaning steps between individual measurements are required, with the result this a high sample throughput per unit of time is achieved. This is of great significance especially for routine analysis or in the field of genetic engineering analysis.
In addition to the analysis of several sample solutions simultaneously, it is also possible for one sample solution to be examined for several of its analytes simultaneously or in succession on one such sensor platform. This is advantageous especially in the case of blood or serum testing which can thus be carried out especially quickly and economically. When several sample solutions are analyzed simultaneously, the separate waveguides prevent cross-talk between luminescence signals from different samples. A high degree of selectivity and low error rates are achieved with this method. The sensor platform further has an advantage in the fact that the individual separate waveguides can be selectively addressed optically, chemically or fluidically.
Other advantageous embodiments will become apparent.